The present invention relates to a method for preventing edge cracks or rough edges in a grain oriented silicon steel sheet produced from a continuously cast slab containing 2.5 to 4.0% silicon, particularly edge cracks or rough edges which are caused during hot rolling of the grain oriented silicon steel sheet for a purpose of increasing the production yield.
Rough edges are defined in the present invention as edge cracks caused during the hot rolling step and these edge cracks are direct causes of coil fractures in the subsequent cold rolling step.
A conventional way for overcoming these defects comprises preventing over-cooling of both edges of the sheet or controlling the tension given to the sheet, both being carried out in the finish hot rolling step.
However, these defects are not found in the ingot casting process, but are quite often caused in the continuous casting process.
Therefore, coil edges having edge cracks must be slit off by a slitter before the cold rolling step, so that the production yield is extremely lowered.
Continuously cast materials containing high silicon contents particularly suffer from sever edge cracks when hot rolled into coils because of their cast structure which is produced in the casting. These edge cracks sometimes extend 200 mm from the coil edges.
The biggest problem of the hot coil which is produced by the continuous casting process is the lowering of production yield caused by slitting rough edge parts. However, no successful measures have been made to overcome these defects in the rolling step.
It is well-known in this field that the grain oriented silicon steel sheet for a transformer contains from 2.5% to 4.0% silicon and also contains at least one or more impurities such as Mn, S, Al, N, Sb, B, Se, Cu for forming a precipitated dispersion phase (hereinafter called inhibitors). These inhibitors are necessary for the secondary recrystallization which contributes to improve the magnetic properties.
In order to dissolve these inhibitors completely in the steel, the slab is generally heated at a high temperature not lower than 1250.degree. C. This slab heating temperature is different depending on the contents of these inhibitors. On the contrary, as a general trend, the higher the slab heating temperature is, the more the edge crack in the hot coil increases.
Therefore, in order to overcome the above described defects, the present inventors studied causes of the edge cracks. We found that these edge cracks are influenced by the pass schedule in the hot rolling step, particularly the reduction rate at the last half stage in rough rolling passes, more particularly in the last one or two rough rolling passes before the hot finishing rolling.